A great many implantable systems for cardiac monitoring and/or therapy comprising sensors located in a blood vessel or heart chamber coupled with an implantable monitor or therapy delivery device have been proposed or implemented. For example, such cardiac systems include implantable heart monitors and therapy delivery devices including pacemakers, cardioverter/defibrillators, cardiomyostimulators, and drug delivery devices. All of these systems include electrodes for sensing and sense amplifiers for recording and/or deriving sense event signals from the intracardiac or remote electrogram (EGM). In current implantable cardiac devices providing a therapy, the sense event signals are utilized to control the delivery of the therapy in accordance with an operating algorithm and at least selected EGM signal segments and sense event histogram data or the like are stored in internal RAM for telemetry out to an external programmer at a later time. In implantable cardiac monitors, e.g., the MEDTRONIC.RTM. Reveal implantable heart monitor, a 42 minute segment of EGM is recorded when the patient activates it, by applying a magnet over the site of implantation, in response to feeling the effects of an arrhythmic episode.
Such implantable medical devices that provide a therapy and/or monitor a physiologic condition or state are programmable and/or can be interrogated by an external programmer through the use of bidirectional RF telemetry that exchanges data and commands via uplink and downlink RF telemetry transmissions through the patient's skin. A great many telemetry schemes have been employed and proposed by the assignee, Medtronic, Inc., that typically involve short range telemetry transmissions employing a 175 kHz RF carrier and close physical coupling of magnetic fields encompassing the RF telemetry antenna coils of the implanted medical device and a programming head placed against the patient's skin. A great many other telemetry systems have been proposed to achieve longer range, yet secure, RF telemetry between implantable and external monitoring devices as described, for example, in U.S. Pat. No. 5,113,869 and in commonly assigned U.S. patent application Ser. No. 08/900,624 filed Jul. 25, 1997, for IMPLANTABLE MEDICAL DEVICE MICROSTRIP TELEMETRY ANTENNA in the names of Weimin Sun et al., both incorporated herein by reference.
In addition, various other communication systems have been proposed to effect communication of data and commands between external, body worn, medical devices and implantable medical devices. In U.S. Pat. Nos. 5,487,752 and 5,540,727, assigned to Cardiac Pacemakers, Inc., systems are disclosed for optimizing the function of an implantable medical device by employing remote sensor modules for measuring parameters related to cardiac performance and deriving programming commands for optimizing the operating modes and parameters of the implantable medical device using a telemetry system providing uplink and downlink RF telemetry transmissions therebetween and through the patient's skin.
Moreover, several systems are disclosed for communicating between primary implantable or skin contacting devices and secondary implantable or skin contacting devices using the body as a communication medium as disclosed in U.S. Pat. Nos. 4,524,773, 4,494,950, 4,987,897, and 5,113,859, all incorporated herein by reference. In certain of these patents, secondary remotely implanted or skin worn physiologic sensor modules are described for sensing particular physiologic conditions or states to derive remote sense signals representative thereof. The sensor modules encode the remote sense signals for transmission and transmit the encoded remote sense signals to the primary implantable medical device for processing and use in an algorithm controlling the delivery of a therapy either by the primary implantable medical device or another secondary, therapy delivery, implantable medical device.
Efforts have also been underway for many years to develop implantable physiologic signal transducers and sensors for temporary or chronic use in a body organ or vessel usable with such implantable medical devices for monitoring a physiologic condition other than or in addition to the EGM to derive and store data and/or to control a therapy delivered by the implantable medical device. In respect to cardiac monitoring, it has been proposed to sense and record such additional physiologic signals including blood pressure in or adjoining blood vessels and heart chambers during the cardiac cycle, blood temperature, pH, and a variety of blood gases. Implantable heart monitors and blood pressure and temperature sensors that derive absolute blood pressure signals and temperature signals are disclosed in commonly assigned U.S. Pat. Nos. 5,368,040, 5,535,752 and 5,564,434, and in U.S. Pat. No. 4,791,931, all incorporated by reference herein. A comprehensive listing of implantable therapy delivery devices are disclosed in conjunction with implantable sensors for sensing a wide variety of cardiac physiologic signals in U.S. Pat. No. 5,330,505, incorporated herein by reference. Numerous attempts have been made over the years to refine implantable blood pressure sensors that accurately reflect the actual changes in cardiac blood pressure as set forth in the above-incorporated, commonly assigned, '752 and '434 patents.
Blood pressure and temperature signal values respond to changes in cardiac output that may be caused by a cardiac failure, e.g., fibrillation or high rate tachycardia, or that may reflect a change in the body's need for oxygenated blood. In the former case, monitoring of a substantial drop in blood pressure in a heart chamber, particularly the right ventricle, alone or in conjunction with an accelerated or chaotic EGM, was proposed more than 30 years ago as an indicia of fibrillation or tachycardia sufficient to trigger automatic delivery of defibrillation or cardioversion shock. More recently, it has been proposed to monitor the changes in blood pressure (dP/dt) that accompany normal heart contraction and relaxation and blood pressure changes that occur during high rate tachycardia and fibrillation or flutter.
A number of cardiac pacing systems and algorithms for processing the monitored mean and dP/dt blood pressure have been proposed and, in some instances employed clinically, for treating bradycardia. Such systems and algorithms are designed to sense and respond to mean or dP/dt changes in blood pressure to change the cardiac pacing rate in a rate range between an upper and a lower pacing rate limit in order to control cardiac output. Similarly, a number of cardiac pacing systems have been proposed, e.g., the system disclosed in U.S. Pat. No. 4,436,092, incorporated herein by reference, and, in some instances employed clinically, that sense and respond to changes in blood temperature to change the cardiac pacing rate in a rate range between an upper and a lower pacing rate limit in order to control cardiac output.
Certain of the measured physiologic signals derived from the heart or blood in the circulatory system are affected by ambient conditions that cannot be separately measured by the implantable medical device. Specifically, blood pressure and temperature signal values derived by a wholly implantable system are affected by atmospheric pressure acting on the patient and ambient temperature or by a fever afflicting the patient, respectively. In addition, certain implantable blood pressure sensors, e.g., those disclosed in the above-incorporated, commonly assigned '434 and '752 patents, are also affected by blood temperature changes.
In commonly assigned U.S. Pat. No. 4,407,296, a pressure sensing lead is disclosed that attempts to account for the affect of atmospheric pressure by providing an air chamber behind the sensor diaphragm exposed to blood pressure that is either sealed at a known average atmospheric pressure or leads to a further membrane or diaphragm near the proximal end of the lead body that is to be positioned in the abdominal cavity where the implantable monitor or pulse generator is implanted. In practice, this approach has proven to be inadequate because the known pressure cannot account for changes in barometric pressure and renders the blood pressure measurements ambiguous and the membrane on the lead body is difficult to manufacture, fragile and can become obstructed in chronic implantation.
In the above referenced '505 patent and in the related U.S. Pat. No. 4,899,751 and other patents by the same patentee, long term and short term mean blood pressure values are derived from the same implantable sensor and combined in an attempt to predict the onset of a cardiac arrhythmia or to provide an indication of the patient's requirements for cardiac output. This approach has not proven to be capable of negating the effects of barometric pressure on the long term and short term mean blood pressure values.
The absolute blood pressure changes, including both mean or average blood pressure and dP/dt pressure changes that are sensed by the implantable pressure sensors are influenced by barometric pressure changes. For example, when a patient such an implantable blood pressure sensing medical device changes elevation by ascending or descending in an elevator in a tall building or in an airplane, the change in barometric pressure changes the absolute blood pressure sensed in the body by an amount that can mask changes that are sought to be measured. In the context of an implantable rate responsive pacemaker operating under a rate control algorithm, the pressure change caused by the elevation change itself may exceed the blood pressure change that reflects a change in exercise level of the patient and be mis-interpreted as meriting a change in pacing rate to the upper or lower pacing rate limit, which can, at least, be uncomfortable to the patient. The barometric pressure effect can similarly have a negative effect on operating and detection functions of other implantable medical devices reliant on accurately sensing cardiac blood pressure changes that truly reflect a cardiac function or requirement for cardiac output.
Barometric pressure acting on the body can also affect the operation of other implanted sensors, e.g., respiration sensors relying on the use of impedance plethysmography. A number of cardiac pacing systems have been proposed and, in some instances employed clinically, for treating bradycardia that sense and respond to changes in respiration as measured by impedance changes between electrodes spaced across the patient's thorax from which minute ventilation is derived. The impedance changes are quantified in time to derive a control signal for increasing or decreasing the cardiac pacing rate in a rate range between an upper and a lower pacing rate limit in order to control cardiac output. The impedance signal baseline and rate of change can be affected by the barometric pressure reflected in the patient's lungs which changes with weather and elevation changes made by the patient.
It has also been proposed to monitor respiration induced pressure waves from sampled absolute blood pressure values and to derive respiration rate therefrom. The sampled absolute pressure signal baseline and rate of change can be affected by the barometric pressure reflected in the patient's heart which also changes with weather and elevation changes made by the patient.
Conceptually, similar problems can accompany the reliance on blood temperature as an indicia of patient activity level, for example. A fever or a high ambient air temperature raising the blood temperature can be mis-interpreted as an indicia of elevated patient activity and be mis-interpreted by a therapy delivery device, e.g., a rate responsive cardiac pacemaker.
At this time, I am not aware of any practical way to measure the ambient air pressure affecting the sensed blood pressure or the ambient temperature affecting the sensed blood temperature and separate it from the internally sensed absolute pressure and temperature. In the context of implantable heart monitors of the type described above for measuring absolute blood pressure, the resulting data may be misleading or inconvenient to interpret by the physician. Physicians are accustomed to taking and interpreting external readings of blood pressure using apparatus that takes barometric pressure into account. For this reason, it is suggested in the above-incorporated, commonly assigned, '752 and '434 patents that the patient may be provided with a belt worn external pressure recorder that records and time stamps recordings of barometric pressure that can be retrieved and used for comparison with the internally recorded absolute blood pressure data.
Despite the considerable effort that has been expended in designing such implantable medical devices and associated sensors for sensing such physiologic signals, a need exists for a system and method for accounting for ambient conditions surrounding the patient that affect the sensed and measured physiologic signal values, particularly in the case of blood pressure and temperature.